cmd/cgo/internal/testtls: cleanup and support more arches
Currently, this test only enabled on non-Darwin UNIX platforms because
it uses the non-standard _thread attribute for thread-local storage.
C11 introduced a standard way to declare something thread-local, so
this CL takes advantage of that to generalize the test to Darwin and
Windows.
Change-Id: Iba31b6216721df6eb8e978d7487cd3a787cae588
Reviewed-on: https://go-review.googlesource.com/c/go/+/496295
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>